In conventional coating hoppers for coating a fluid onto the surface of a web, such as hoppers for photographic emulsions and polymeric materials, fluid is delivered into the hopper through a center inlet passage or an end inlet passage. In either case, the inlet passage is in fluid communication with an extrusion slot portion of the hopper via an interior flow distribution manifold or channel formed in at least one of the die portions comprising the hopper. In both cases, achieving relatively uniform widthwise flow distribution at the extrusion slot is a problem. One way known to accomplish more uniform widthwise flow distribution is to contour specially the interior flow distribution manifold and lips of the extrusion slot. Contouring of the manifold is needed to offset the pressure loss which must occur between the inlet passage and the far end of the manifold. Properly done, this conventional practice is adequate for a specific set of fluids and flow conditions. However, a shortcoming of this practice is manifest when there is a change in fluid flow. Typically, a change in fluid flow alters the pressure relationship in the manifold to create unacceptable flow distribution, resulting in excessive fluid accumulation or flooding in the far end of the manifold.
Rather than contouring the manifold, a different flow path or flow circuit has been shown to evenly distribute the pressure of the fluid flow in the hopper. One such flow circuit is the criss-cross circuit in which fluid enters the inlet channel and crosses as it enters the inlet channel. However, the added complexity of the passages needed in the hopper device does require some special method of hopper construction while maintaining the mechanical stability of the extrusion slot.
The use of some sort of insert device to achieve flow uniformity of a single stream has been attempted in the prior art. U.S. Pat. No. 4,017,240 to Nelson teaches sheet extrusion of elastomeric material wherein an extruded strand is passed through a die having a symmetrical runner system which divides and subdivides the strand into multiple strands equal in length and cross section. The strands are then gradually transformed into laterally adjoining flattened triangular portions in which substantially the same cross-sectional area is maintained as the cross-sectional shape changes. The shaped strands then pass out of the manifold in a continuous lateral exit. This device, therefore, has the shortcoming of branching the material flow through an array of channels which all empty into a final damping cavity.
U.S. Pat. No. 4,411,614 to Feathers teaches a removable contoured insert for an extrusion die dedicated to a particular coating fluid. The insert comprises a plurality of feed passages adapted to communicate with the distribution channel and with the inlet passage of the die. Fluid passing through the plurality of feed passages, similar to Nelson '240, then empties into a final damping cavity.
Accordingly, there persists a need for an apparatus for achieving uniform flow distribution in a coating hopper for emulsions and polymeric materials that obviates the shortcomings of the prior art.